Electron discharge apparatus



' March 12, 1940. E, BR CE 2,193,579

ELECTRON DISCHARGE APPARATUS Filed Jan. 26, 1938 I 3 Sheets-Sheet 1 m ummm Ill i n 'upm' 4L Y i; *5

ANODE CURRENT IN MILL/AMPEPES -|2--|o -s -a -4 -2 o CONTROL VOLTAGE WHENANODE I25 u ACCEL 4.0 K CONTROL =-5.5 l!

ANODE CURRENT IN MILL/AMPERES 5 CONTROL 0 A ELE TR 0 2o 40 so so I00 I20I40 I60 47 ANOOE VOLTAGE INVENTOR ANODE E. BRUCE ACCELERAT/NG ELECTRODEc4 THODE Walla?! 6 AT TORNEV ACCELERA TING ELECTRODE s Sheets-Sheet 2FIG. I? I CAT/{ODE FIG. /4

E. BRUCE 8 0 K JO I20 INVENTOR E. BRUCE ATTORNEY ANODE VOLTAGE 04am a.14-24% OUTPUT wwkmniifii 3 K553. meek m w a 6 4 2 o ELECTRON DISCHARGEAPPARATUS Filed Jan. 26, 1938- FIG. /3

March 12, 1940.

FIG. 4

INPUT CONTROL VOLTAGE March 12, 1940. E. BRUCE 2,193,579

ELECTRON DISCHARGE APPARATUS Filed Jan. 26, 1938 S Sheets-Sheet 5 ANODECURRENT IN MILLIAMPERES Avon: +230 u ACCEL. [6 u 5 1 I I l 4 -|zo no.0a.o -s.o .o -z.o o

colvmb'L VOLTAGE waemajuwa A 7' TORNEV Patented Mar. 12, 1940 ELECTRONDISCHARGE APPARATUS Edmond Bruce, Red Bank, N. 'J., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a'corporation of-New York Application January 26, 1938, Serial No. 186,950

zoo-aims. (01. 250-27) This invention relates to electron dischargeapparatus and more particularly to such apparatus A as disclosed in myapplication Serial No. 134,008, Vfiled March 31, 1937, of whichthisapplication is in part a continuation, including an electron dischargedevice of the electron beam type.

In the specific form of electron discharge apparatus disclosed in myprior application above identified, the electron discharge devicecomprises 16 a cathode, a pair of spaced grid electrodes surrounding thecathode, and an anode outside of theoutergrid electrode. Each of thegrid electrodes comprises a plurality of spaced linear ele-. v ments,parallel to each other and to the cathode, m the corresponding elementsof the two grid electrodes being in radial alignment with one anotherand with the cathode. The output electrode or anode includes a pluralityof linear elements mounted parallel to the cathode, each of these 20anode elements being in alignment with thecathode along agradiusbisecting the space between successive corresponding elements ofthe gridelectrodes. 1 In such apparatus, it was found that forspecie fie dconditionsof operation, the control grid potentiakanode. currentcharacteristic included a region wherein a negative transconductance obtained, thatis, a region wherein'the anode cur-' also as the ratio ofthe amplification factor to If the transconductance is the anoderesistance. to be negative, oneof two possibilities must obtain,namelveither a positive amplification factor vw anda negative anoderesistance or a negative amplification factor and a positive anoderesistance. One general object of this invention is to obtain a negativetransconductance in electron discharge d'evicesp I 15 More specifically,objects of this invention are: To increase the magnitude of the negativetransconductance in electron discharge devices having a linear anode oroutput electrode and a multiele- 1 went control electrode or grid; I

a negative anode resistance in electron discharge devices, withoutsecondary emission from the anode; i To increase the range of grid orcontrol elecelement.

To obtain a positive-amplification factor and trode potentials wherein anegative transconduct ance is'exhibited; and I Toobtain a plurality ofspaced regions in the operating characteristic of electron dischargedevices wherein the anode oroutput current de-, 5 creases as thecontrol-electrode or grid potential becomes less negative. 1

In one illustrative embodiment of this invention, electron dischargeapparatus comprises an electron discharge device having a cathode, a 10plurality of linear anode elements disposed about the'cathode andparallel thereto .and to one another, and a control electrode or grid.The control electrode or grid comprises a plurality of linear rods orwires so mounted that each anode 15 element has thereabout a pluralityof these rods or wires located in an arcuate boundaryat least partiallyencompassing the corresponding anode The constant potentials upon thevarious electrodes are such that, throughout a defi- 3 nite rangeofgridor control,electrode potentials,

" the output or anode current'decreases asthe grid The invention and thefeatures thereof will be understood clearly and fully' from thefollowing detailed description with reference to theaccompanyingdrawings in which: I

Fig. .l is a perspective viewof an electron discharge deviceillustrative of one embodiment of 3Q this invention, aportionottheenclosing vessel and of theelectrode structure being brokenaway to show the inner electrodes more clearly; v

. Fig. Zis a view in section, and partly diagrammatic, of the electrodesin the device s'hown in Fig. 1, illustrating the arrangementand-electrical association of the various. electrode elements;

Fig. Sis a diagrammaticfragmentary view of aportiono'f the electrodes inthe device illustrated in Figs. 1 and 2, showing clearly the relativepo- 40 sitions of the anode and control electrode or grid j elements; Ir v Fig; 4 is a circuit diagramz illustrating a typicalcircuitalassociation of the electrodes of theelec-f tron dischargedevice shown in Fig. 1;

5 and 6 are g'raphsshowing .the relaticnship between the controlelectrode or grid potene tial and the output or anode currenhand betweenA the anode voltage and anode current respectively,

of'an electron discharge device of the construc,- -5o 1 tion illustratedin Fig. 1, connected incircuit as illustratedin Fig. 4;

Fig. 7 is a perspective view of an electron disv charge deviceillustrativeof another embodiment r of this. invention; Y I v Fig. 8 isa view partly diagrammatic and partly sectional illustrating thearrangement and electrical association of the electrode elements in theembodiment of this invention illustrated in Fig. 7;

Fig. 9 is a diagrammatic fragmentary view showing clearly the spacerelation of the anode and control grid elements in the embodiment ofthis invention illustrated in Fig. 7;

Fig. 10 is a graph illustrating a typical control electrodepotential-anode current characteristic of an electron discharge deviceof the construction illustrated in Fig. '7;

Fig. 11 is a view partly diagrammatic and partly sectional of theelectrodes in an electron discharge device illustrative of still anotherembodiment of this invention;

Fig. 12 is a diagrammatic fragmentary view showing clearly the relationof the electrode elements in the electron discharge device illustratedin Fig. 11; and

Figs. 13 and 14 are graphs showing typical control electrodepotential-anode current and anode voltage-anode current characteristicsrespectively, of an electron discharge device of the constructionillustrated in Fig. 11.

Referring now to the drawings, the electron discharge device shown inFig. 1 comprises an evacuated enclosing vessel 20 having a stem 2| atone end and having suitably aflixed thereto a base 22 carrying terminalprongs 23 through which the electrodes of the device may be associatedwith external circuits.

Embedded in the press 24 of the stem 2| and arising therefrom areparallel, rigid, metallic supports or uprights 25, one of which isconnected electrically to one of the terminal prongs 23 by a leading-inconductor 26. An insulating disc 21, for example, of mica, is affixedtothe supports or uprights 25 adjacent the upper end thereof, as byclips 28 which may be welded to the uprights or supports. Similar spacedinsulating discs 29 and 30, which also may be of mica or the like, areaffixed to the supports or uprights 25 by suitable clips 3|, which alsomay be welded to the uprights or supports.

An elongated linear cathode, which may be either a filament or of theequipotential heater type, is supported between the insulating discs 21,29 and 30. In the form shown in Figs. 1 and 2, the cathode comprises acylindrical metallic sleeve 32, coated on its outer surface with athermionic material, one end of which extends through and is fitted incentral apertures in the lower insulating discs 29 and 30 and the otherend of which is reduced, as indicated at 33, and fitted in a centralaperture in the upper insulating disc 21. The cathode sleeve 32 enclosesa heater filament 34 embedded in or threaded through a suitable ceramicor insulating body 35. Heating current may be supplied to the filament34 through leading-in conductors 36 embedded in the press 24 andconnected. to corresponding ones of the terminal prongs 23. Electricalconnection to the cathode 32 may be established through a leading-inconductor 31 also embedded in the press 24 and connected to one of theterminal prongs 23.

The cathode 32 is surrounded by a cylindrical accelerating electrode orgrid coaxial therewith and including a plurality of equally spacedlinear conductors or wires 38 mounted parallel to one another and to thecathode. The conductors 38 extend. through aligned apertures in theinsulating discs 21, 29 and 30 and are electrically connected to oneanother by a metallic band or collar 39 seated upon the insulating disc30. One of the conductors may be connected to one of the terminal prongs23 through a leading-in conductor 4|] embedded in the press 24.

The accelerating electrode or grid 38 is encompassed by a controlelectrode or grid, preferably coaxial therewith, which comprises aplurality of parallel, linear conductors or wires 4| each of which, asshown clearly in Fig. 2, is in radial alignment with the cathode 32 andalternate of which are in alignment also with a cor-- responding one ofthe wires 38 of the inner or accelerating grid. The conductors or wiresll extend through aligned apertures in the insu-' lating discs 21, 29and 30 and are connected together electrically by a metallic band orcollar 42 seated upon the insulating disc 30. Suitable potentials may beapplied to the control electrode through a leading-in conductor 43connected to one of the conductors or wires 4| and to one of theterminal prongs 23.

The control electrode or grid 4| encompasses an anode, coaxialtherewith, which comprises a plurality of equally spaced parallel,linear rods or wires 44'. The anode wires or conductors 44 extendthrough aligned apertures in the insulating discs 21, 29 and 30 and areelectrically connected by a metallic band or collar 45 seated upon thedisc 3!]. The anode may be coupled to an external circuit through aleading-in conducto-r 46 connected to one of the terminal prongs 23 andto one of the conductors or wires 44.

The control electrode or grid may be encompassed in turn by an auxiliaryelectrode having a cylindrical portion 41 and diametrically oppositeflanges 48 which may be secured, as by welding. to the uprights orsupports 25. The auxiliary electrode 4! may be connected directly to thecathode by a conductor 54.

As will be seen clearly fromFigs. 2 and 3, each of the elements 44 ofthe anode has cooperatively associated therewith three elements 4| ofthe control electrode or grid, which lie in an arcuate boundary,indicated by the dotted line A, partially encompassing thecorrespondingv anode element 44 and preferably coaxial with the oathode.The device, then, includes in effect six sections of electrodeselectrically connected in parallel. v

In a specific embodiment, the grid and-anode elementsmay be rodshaving adiameterof 0.015", the cathode may have a diameter of 0.067, and theelements may be spaced so that the electrode spacings are as follows: I

Radius,

inch Accelerating electrode to axis 0.065 Anode to axis 0.125 Controlelectrode to axis 0.185 Cylindrical plate to axis 0.250

During operation of the electron discharge device illustrated in Figs. 1to 3, inclusive, as shown in Fig. 4, an input circuit including anetwork 49 and a source, such as a battery 50, for applying An outputcircuit 5| may be con-- anes-p79 it is at a positive. potential,lowerhtha'n the an ode potential, with respect'to the'cathode.-. Pref-eerably, "the fixed potentials applied to the'several electrodes are suchthat the electrons emanating from the cathode are concentratedin'circula-r 'or .elliptical paths about the anodewires Hi. .Theauxiliary electrode 41, nOlLShOWIi-ill Fig. Jamey be connected directljto the cathodeas described hereto-fore and operated at cathodepotential.

When the electron discharge device illustrated in Figs. 1 to 3,inclusivais connected in'circuit as shown in and the anode and acoelerating electrode are maintained at positive potentials with respect to thecathode (for example, 125;

volts and 4.0 volts, respectively, in the specific embodiment describedhereinabove) it has been found. that the anode current varies withrespect to negative control electrode or grid potential in themannershown by curve-B inFig'. 5.- From this curve it will be seen that as thepotential upon the control electrode or. grid is made less negative, theanode current decreases.

At the value of anode potentials given, as indicated by the point-Conthe anode current-anode voltage characteristic D in Fig. 6 (thecontrol electrode being biased at 5.5 volts negative with respect to thecathode), the anode resistance (RA) is positive. Hence, there being anegative I transconductance (Sea) as indicated by the slope of the curveB in Fig. 5,'the amplification fac tor (,IL) necessarily is negative.

may be noted that forthe potentials and param- Specifically, it

etcrs given hereinabove, at an anode potential indicated by point 'C(125 volts), the anode resistance is approximately 10,000 ohms,positive,

and the amplification factor is substantially 16, negative. Hence, the.mutual conductance is sub stantially 400o 10 mhos, negative.

If the anode potential is'increased and the grid and acceleratingelectrode .potentials are. held constant, it has been'found that,asindicated by iii) tive.

the portion F of curve D,the anode resistance becomes negative for arange of anode potentials,

and then becomes positive again as indicated by the portion G of thecurve D. Inasmuch as the anode is highly positive with respect totheoth'er electrodes of the device, it may be pointed out that thisnegative resistance is not attributable to secondary. electron'emissionfromthe anode.

If the anode is held at a constant potential in the range wherein anegative anode resistance obtains, the anode current varies-with thepotential of the control electrode or grid as indicated by the curve Hin Fig. 5 from which it will be seen that the transconductance, isnegative. Hence, the amplification factor is positive. Specifically, foran anode potential of 138 volts, a negative controlelectrode potentialof 5.5 volts, and a positive-accelerating electrode potential. of

'4 volts, the anode resistance of the device described aboveis aboutL000 ohms negative and the amplification factor is substantially 40,posi- Hence, the transconductance .is approxi-v mately 10,000xl0-,mhos,- negative.

It may be pointed out that when the potentials upon the electrodes ofthe electron discharge 'device are such that the anode resistance ispositive, the load resistance may beselected in the usual manner toattain a desired efficiency. However, Whenthe potentials are such thatthe anode resistance-is negative, theinsertion ofa small positiveresistance in the load oroutput circuit, in series with. thenegativeanode resistoperating potentials may 'be necessary. a:

. i l feet obtains throughout a relativelywide range of controlelectrode or grid potentials.

. 3 ance, reduces the resultant negative resistance and increases theapparent negative transcon duct'ance of the device. Hence, specialdetermi nation of the load resistance for the particular In theembodiment of this invention illustrated in Figs. '7, 8 and 9, theform,number and arrangement of the cathode, auxiliary electrode,

and the various linear electrode elements are the same as that in theembodiment shown in Fig, 1. Certain of. the elements, however, areassociated difierently electrically. In Figs.-7, 8 and 9,,the elementsof the electrode structure corresponding mechanically orelectrically tothose in the embodiment shown inFig. l are designated b'y'thecorresponding reference numeral increasedrb-y 10.0. I I I As shown inFig.1 the electrode structure includes anadditional insulating spacerI55, such as *a, mica disc, maintained in parallel relation to theinsulatingspacer I27 by metallic clips: I28. afiixed to the uprightsI25. Mounted uponthe insulating spacer I55 is a metallic collar 'or band0 nected, either internally or externally'of thetenclosing. vessel, tothenext inner members,v as, indicated-by the connection I42, M5 in Fig.8, and constitutethe elements MI of the control electrode or'grid,mounted at the apices of the angles, of a rsix pointed star-shapedboundary. ,.-The auxiliary electrode I41 .is connected totheelementsi MIby a tie wire IHl andconstitutesa part of the control electrodeofythedeviceu Each of theanode'elements' It'll. has cooperatively asso-3 ciated therewith three of, the elements. MIjof-the controlelectro'de01' grid, which lie in an 'arcuate boundary, indicated by the dottedline A in Fig. 9, partially encompassing the corresponding anode elementIM. 1 The radius of the boundary A may beapproximately equaltothedistance elements M4. I

The-jelectron discharge device illustratedin Figs, 7, 8 and 9 may beconnected in circuit as shown in Fig. 4. "Iheelectrode parameters may besubstantially the same as'those-given herein-' above for the-specificembodiment of; the device illustrated in Fig. 1. When thus-connected,and

with the anode and acceleration electrode maintained at constantpositivepotentials', "for ex ample, 'theanode at a' potential of 230volts and theqaccelei'ation electrode-at a potential of '16 9+ betweenthe centers of thecathode Itzfand anode Iii)- volts,; the anode currentvaries with theflcontrol electrode or grid voltage as-indicated bytheillustrative curve in'Fig: 1%). As will be apparent irom this curve,-the ,transconductan'c'e of the electrondischarge device. is alternatelypositiveand negative as indicated by the portions- J and-"J and K and K?respectively. At points X and "X corresponding, respectively tonegativercontrol electrode potentials of 8.5 and 3.5 volts,thetransconductance may he'- otthe .order of 700x10- mhos, negative, andthe anode resistance'may be negative andl-AOO ohms, and 33,000 ohms,respecitively. Theamplificationfactor, then'is Q positive, "at the pointX and 23, positive, 'at'-the point X a 'From Fig. 10 it will be apparentthat if the variable potentials'impre'ssed upon the control electrode orgrid by the input circuit 49 are of suflicient magnitude to embrace theregion including J, X and X each pulse or swing of the input potentialwill produce at least four pulses'in the output current so that inefiect a frequency quadrupling is obtained. The magnitude and frequencyof the output current will be dependent,

of 'course, upon the electrical and mechanical I parameters of theelectron discharge device.

7 The negative transconductance, with either positive or negative anoderesistance, may be obtained also by 'connecting the various elements ofthe electron discharge device shown in Fig. '7 in the manner illustratedin Figs. 11 and 12. In these figures the elements correspondingmechanically or electrically to those'of the embodiment illustrated inFigs. 7, 8 and 9 are designated by the same characters as in the latter,increased by 100.

As shown in Figs. 11 and 12, the anode-comprises alternate of theoutermost linear members 244-electrically connected by the collar orband 256 and the control electrode or grid comprises the other outermostlinear members 24l electrically connected by the collar or band 245. Thegrid composed of the intermediate linear members 260, connected by thecollar or band 2.62, is connected directly to the cathode 232 by theconductor 254.

The elements 24| lie at the corners of a regular hexagon and eachsuccessive two of them lie in an'arcuate boundary; indicated by thedotted line A in Fig. 12, partially encompassing the corresponding anodeelement 244, are equally spaced from such anode element, and'are spacedthe same distance from the cathode 232 as the anode elethe parametersthereof may be the same as those given heretofore for the specificembodiment of the device'illustrated in Fig. 1. When thus operated andwith constant positive potentials upon the anode and acceleratingelectrode, for example, 100'volts' and 14 volts, respectively, the anodecurrent varies with the negative control electrode voltage as indicatedby the. curve M in Fig. 13, from which it will be apparent that anegative trarisconductance obtains throughouta relatively wide range ofcontrol electrode potentials. .If the control electrode is maintained ata fixed negative potential, for example 6 volts, and the acceleratingelectrode is maintained positive, for example 14 volts, the anodecurrent varies with anode potentialas indicated by the curve N inFig.'14, from which it will be seen that the anode resistance is,positive. Hence, the amplification factor is negative; for example, atthe values of potential given, that is if anode voltage equals 100volts, control electrode potential equals '6 volts and acceleratingelectrode potential equals 14 volts, the amplification factor may be 6.5negative, and the transconductance 500 10 mhos,

, negative;

If the control electrode is maintained at a fixed potential, for example6 volts negative, and the accelerating electrode is made less positive,for example 9 volts, the anode current varies with anode voltage asindicated by the curve 0 in Fig. 14,'from which it will be seen that fora range of anode voltages, the anode resistance is negative. If theanode potential is maintained at 'a fixed value in this range, forexample 100 volts, the anode current varies with control electrodepotential as indicated by the curveP in Fig. 13. At the pointY on thiscurve, corresponding to a negative control electrode potential of 6volts and a negative [anode resistance of 25,000 ohms, thetransconductance is about 500 10 mhos, negative, and the amplificationfactor is approximately 12.5 positive.

It willbe apparent. from Figs.l3 and 14 that in the device illustratedin Figs. -11 and 12, a negative transconductance, with either a positiveanode resistance and negative amplification factor or a negative anoderesistance and positive ratus constructed in accordance with thisinvention may be utilized advantageously in a variety of applications.In addition to the application in frequency multiplying systemsmentioned heretofore, the invention may be utilized, for example, inamplifiers, with or without negative feedback, in detectors or inoscillators.

Although specific embodiments of this inventrode. an anode including alinear electron receiving element, said control electrode includingspaced linear elements mounted about said electron receiving element, aninput circuit connected to said cathode and said control electrode, andan. output circuit connected to said cathode and said anode includingmeans applying such posh tive potentials to said anode and saidaccelerating electrode that the anode current varies inversely withincrements in the anode potential in .the absence of secondary emissionfrom said anode.

2. Electron discharge apparatus comprising a cathode, a linear anodeparallel to said cathode, an accelerating electrode between said cathodeand said anode, a'control electrode including a pair of linear elementson opposite sides of said anode, an input circuit connected to saidcathode and said control electrode including means for applying anegative bias to said control electrode,

. means for applying a positive potential to said acceleratingelectrode, and an output circuit connected to said cathode and saidanode including means for maintaining. said anode at a positivepotential higher-than the potential of said accelerating, electrode,said bias and said anode and accelerating electrode potentials beingsuch that the anode current varies inversely with incretrode including aplurality of linear elements disposed about said accelerating electrodeand in cooperative relation with said electron receiving elements, aninput circuit connected between said cathode and said control electrode,means applying a positive potential to said accelerating electrode, andan output circuit connected between said cathode and said anodeincluding means maintaining said-anode at a positive potential higherthan the accelerating electrode potential, said positive potentialsbeing such that throughout a range of increments in the anode potentialthe current in said output circuit varies inversely with anodepotential.

5. Electron discharge apparatus in accordance with claim 4 wherein saidanode and control electrode elements are mounted parallel to one anotherand said cathode and are substantlall equally spaced from said cathode.

6.- Electron discharge apparatus comprising a cathode, an anode, and acontrol electrode including a plurality of linear elements mounted 8.Electron discharge apparatus comprising a cathode, a linear anode inalignment with said cathode, an accelerating electrode including alinear member between said cathode and said anode and in alignmenttherewith, and a control electrode including linear members on oppositesides of said anode.

9. An electron discharge device comprising a linear cathode, a linearanode in alignment with said cathode, an accelerating electrodeincluding a linear member between said cathode and said anode and inalignment therewith, and a control electrode including a member betweensaid accelerating electrode and said anode and in alignment therewithand other members on opposite sides of said anode.

10. An electron discharge device comprising a cathode, a rod anode inalignment with said cathode, and a control electrode including aplurality of linear elements parallel to said cathode, at least two ofsaid elements being spaced from said cathode a distance substantiallyequal to the spacing between said cathode and said anode and disposed onopposite sides of said anode.

11. Electron discharge apparatus comprising a cathode, an anode inalignment with said cathode, and a control electrode including alinearmember in alignment with said cathbde and said anode and other membersequally spacedfrom and on opposite sides of a line passing through saidcathode and the center of said anode.

12. Electron discharge apparatus comprising a cathode, an anodeincluding a plurality of linear electron receiving elements mountedabout said cathode, and a control electrode including a plurality oflinear members between said cathode tween said electron receivingelements.

' 13."Electron discharge apparatus comprising a cathode, an anodeincluding a plurality of linear elements mounted in a cylindricalboundary about said cathode, and a control electrode including aplurality of linear elements mounted in a cylindrical boundarybetween'said cathode and said first boundary, each of said second eleaplurality of electrode elements'mounted in a cylindrical boundary aboutsaid first boundary,

means electrically connecting one group of alternate of said secondelements to said first electrode elements, means electrically connectingthe other alternate of said second elements, an input circuit connectedto said cathode and saidfir'st electrode elements and said group ofsecond elements, and an output circuit connected to said cathode andsaid other alternate second elements;

16. Electron discharge apparatus in accordance with claim 15 comprisingmeans included in said output circuit for maintaining said otherelements at a positive potential, and means in'said input circuitapplying such negative potentials to said first electrode elements andthe group of said second'elements connected thereto that the current insaid output circuit increases as said negative potential increases. 17.Electron discharge apparatus in accordance with claim 15 comprising anaccelerating elecanode, and including spaced elements in an arcuateboundary about said anode. I

19. Electron discharge apparatus comprising a cathode, an anodecomprising a plurality of linear elements mounted in a cylindricalboundary about said cathode, a control electrode comprising a pluralityof linear elementsmounted in a cylindricalboundary about said firstboundary,

said first and'second elements being substantially.

parallel to one another and to said cathode,'an

1 input circuit connectedto said cathode and said control electrode, andan output circuit connected to saidcathode and sai-danode. I

20. Electron discharge apparatus comprising a cathode, a linear anode inalignment with said cathode, anda control electrode including linearelements mounted in an arcuate boundary passing between said cathode andsaid anode and an ele ment mounted outside of said anode andelectrically connected to said linear elements.

EDMOND BRUCE. I

